Epidemiologic Analyses of 

Virulent Newcastle Disease in 

Backyard Birds in California

December 2018 

United States

Department of

Agriculture

Animal and Plant

Health Inspection

Service

Veterinary Services
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

ii USDA APHIS VS

Suggested bibliographic citation for this report: 

USDA-APHIS (2018). “Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds 

in California, December 2018.” USDA:APHIS:VS:STAS:Center for Epidemiology and Animal 

Health. Fort Collins, CO. December 2018. M&M Doc #447.0718. 41 pgs.

For more information, contact: 

Amy Delgado, DVM PhD

Director, Monitoring and Modeling

Center for Epidemiology and Animal Health

USDA:APHIS:Veterinary Services

Center for Epidemiology and Animal Health

2150 Centre Avenue, Bldg. B-2E7, Fort Collins, CO, 80526-8117

Phone (970) 494-7276 | Mobile (970) 420-2567 | Fax (970) 494-7228

Email: Amy.H.Delgado@aphis.usda.gov

Web: http://www.aphis.usda.gov/animal-health/ceah

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offices/veterinary_services/ceah.shtml

M&M Document #447.0718
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Contents

Executive Summary.............................................................................................................................2

Introduction........................................................................................................................................4

A. Disease Overview.....................................................................................................................................4

B. Description of Outbreak ..........................................................................................................................5

C. References...............................................................................................................................................6

II. Phylogenetic Analysis and Diagnostics..........................................................................................9

A. Virulent Newcastle Disease Virus............................................................................................................9

Methods ......................................................................................................................................................9

Results.........................................................................................................................................................9

B. Comparison to Other Viruses/Lineages.................................................................................................10

C. Diagnostics.............................................................................................................................................10

III. Population at Risk......................................................................................................................11

A. Predicting Areas of Backyard Bird Ownership.......................................................................................11

Methods ....................................................................................................................................................11

Results.......................................................................................................................................................11

Summary ...................................................................................................................................................13

B. References.............................................................................................................................................14

IV. Epidemiologic Risk.....................................................................................................................15

A. Case Control Study.................................................................................................................................15

Methods ....................................................................................................................................................15

Results.......................................................................................................................................................15

Summary ...................................................................................................................................................17

Summary of Historical Epidemiologic Risk Factors....................................................................................17

B. References.............................................................................................................................................18

V. Estimating Disease Spread .........................................................................................................20

A. Between Flock Disease Spread Model – Early Outbreak.......................................................................20

Methods ....................................................................................................................................................20

Results.......................................................................................................................................................20

B. Comparing Alternative Control Strategies – Mid-Outbreak ..................................................................22

Methods ....................................................................................................................................................22

Results.......................................................................................................................................................23

C. Within Flock Transmission Model .........................................................................................................24

Methods ....................................................................................................................................................24

Results.......................................................................................................................................................25

D. Spatial and Spatiotemporal Patterns of the Outbreak ..........................................................................25

Methods ....................................................................................................................................................26

Results.......................................................................................................................................................26

Conclusions................................................................................................................................................29

E. References.............................................................................................................................................30

VI. Acknowledgements ...................................................................................................................32

Appendix A: Questionnaire................................................................................................................33

Appendix B: Modeling Scenario Design ..............................................................................................38
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List of Figures and Tables

Figure 1. Counties with confirmed findings of virulent Newcastle disease from 17 May 2018 to 9 

November 2018 in the United States.............................................................................................................7

Figure 2. California vND weekly case detection curve based upon the date the case definition was met for 

a presumptive positive flock by day from 17 May 2018 to 9 November 2018. .............................................8

Figure 3. Preliminary socio-demographic variables used in the model to predict the presence of backyard 

poultry in a census block in southern California. .........................................................................................12

Figure 4. Predicted number of backyard flocks by census block group in southern California. .........................13

Figure 5. Cumulative number of confirmed and presumptive positive vND premises detected in California 

from May 16 to August 25, 2018; data are aggregated at the census block level.......................................27

Figure 6. Location of spatial (upper) and spatiotemporal (lower) clusters of vND in California from May 16 

to August 25, 2018. ......................................................................................................................................28

Figure 7. Time period of occurrence of spatiotemporal clusters of vND in California from May 16 to

August 25, 2018. Colors shown relate to the lower part of Figure 6. ..........................................................29

Table 1. Number of vND confirmed positive premises by California counties and dates of earliest 

confirmation in each county as of 9 November 2018. ...................................................................................8

Table 2. Characteristics of backyard premises that are cases (confirmed/presumptive positive for vND) or 

dangerous contact (DC) premises or control premises (C). Odds ratios (OR) and p-values calculated by 

univariate logistic regression (dangerous contacts excluded). ....................................................................19

Table 3. Adjusted odds ratios (OR) for significant risk factors identified in multivariate regression 

analyses. .......................................................................................................................................................19

Table 4. Qualitative summary comparison of the four alternative response options examined. ......................23

Table 5. Spatiotemporal clusters of vND cases in California from May 16 to August 25, 2018..........................29
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EXECUTIVE SUMMARY

In May 2018, a backyard-chicken owner brought several ill exhibition chickens to a veterinary 

clinic in southern California. The birds were displaying signs of virulent Newcastle disease 

(vND). Biological samples were collected from the chickens and sent the California Animal 

Health and Food Safety (CAHFS) Laboratory where vND virus was detected. The National 

Veterinary Services Laboratories (NVSL) confirmed vND in these birds on May 17, 2018. As of 

November 9, 2018, 175 backyard flocks had been confirmed as infected with the virus.

Once initial response efforts were in place, the U.S. Department of Agriculture’s (USDA) Animal 

and Plant Health Inspections Service’s (APHIS) Veterinary Services initiated a series of 

epidemiologic investigations and studies, which were undertaken collaboratively with bird 

owners, State agriculture personnel, and the USDA’s Agricultural Research Service (ARS). This 

report provides the most current findings to-date and is intended to provide a better 

understanding about how the vND virus is introduced and transmitted. 

The outbreak predominantly affected backyard chickens in an area crossing four Southern 

California counties: San Bernardino, Riverside, Ventura, and Los Angeles. Preliminary genetic 

analysis supports a single introduction followed by secondary spread. Lack of epidemiologic 

data regarding the index premises, and temporal and geographical gaps in available genetic 

data, contribute to the uncertainty surrounding the origin of the outbreak.

These affected counties have a high density of backyard flocks, but such flocks are not typically 

registered and their exact locations are unknown. Using a Bayesian hierarchical model, 

previously identified socioeconomic and demographic variables found to be associated with 

urban poultry ownership were used to estimate the probability of backyard flocks in this area.

Results suggest that a single 10-km zone could have as many as 4,000 backyard flocks, and that

the greater Los Angeles area might have more than 11,000. Modeled backyard ownership maps 

will help inform ongoing surveillance and response efforts.

Analyses of surveys conducted at case, control, and dangerous contact premises1 identified 

flock size, ownership of exhibition birds, high proportions of roosters in flocks, and the use of 

housing that allows contact with wild birds, all of which were determined to be risk factors for 

vND in this population. The percentage of premises reporting the use of Newcastle vaccine was 

low overall. Vaccination of backyard birds is a concern due to the potential for improper 

administration that may lead to development of reservoirs of vND. 

Initial results from disease-spread and control simulations suggested that local disease spread 

would become increasingly important as the outbreak increased in size. This type of disease 

spread is distance-dependent and represents mechanisms of spread that are difficult to trace,

such as movement of free ranging birds, wildlife, or fence-line contact. Good biosecurity 

practices and measures are the best way to reduce local spread, but completely preventing this 

 

1 Dangerous contact premises are defined as premises with backyard birds that are high risk due to either an 

epidemiologic link or proximity to infected premises.
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type of spread is difficult. Overall, the likelihood of disease spillover into commercial flocks is 

low, but spillover was observed in 7 percent of simulated outbreaks. As the outbreak 

progressed, modeling scenarios were developed to compare alternative control options and 

resource levels for response. These scenarios found that minimal response levels (including low 

surveillance and depopulation capacities) were unlikely to prevent continued disease spread in 

backyard flocks. Rapid and targeted surveillance, depopulation, and disposal were most 

effective at minimizing outbreak size and severity. The largest and longest simulated outbreaks 

frequently involve significant disease spread within Los Angeles County, irrespective of the 

selected response option.

Using experimental data available from peer-reviewed literature and unpublished data 

provided by the USDA, ARS, Southeast Poultry Research Laboratory (SEPRL), analysts 

estimated the mean latent period for this virus to be 0.40 days, and the mean infectious period

to be 4.33 days. Using these values, we estimate the time to detect vND in an unvaccinated, 50-

bird backyard flock based on observation of increased mortality (two or more dead birds within 

a 3-day period) to be from 4 to 7 days.

The identification of significant spatial and spatiotemporal clustering patterns of vND in 

California from May to August 2018 supports control strategies of targeting high risk areas for 

disease spread with enhanced surveillance and depopulation activities. The results of this 

analysis identified specific geographic areas, at the census block level, within four vND control 

areas of significant spatial and spatiotemporal disease clustering, particularly in San 

Bernardino and Los Angeles Counties. These areas were or have since been identified as targets 

for enhanced response activities. 
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INTRODUCTION

California and USDA-APHIS have initiated epidemiologic and genetic investigations in response 

to the virulent vND outbreak in backyard chickens in Southern California. These investigations 

will provide a better understanding of factors associated with vND virus transmission among 

backyard chickens and other susceptible species. These investigations include the following:

• Analysis of the phylogenetic characteristics of the virus

• Estimation of the probability of homes in Southern California owning backyard birds

• A field-based study of backyard case and control premises using data collected through 

site visits and interviews with backyard-chicken owners

• An epidemiologic disease-spread simulation model of vND spread among bird-owning 

households in Southern California and comparison of alternative control options

• An examination of within-flock disease transmission and the impact on the time to 

detection in unvaccinated backyard flocks

• An analysis of spatial and spatiotemporal patterns of disease

This report includes the preliminary results from these investigations, in an effort to provide 

producers, industry, and other stakeholders with epidemiologic information and to archive the 

analytical work performed to support outbreak response. 

A. Disease Overview

Newcastle disease is the cause of regular, frequent poultry epizootics throughout Africa, Asia, 

Central America, and parts of South America. The disease is caused by strains of avian 

paramyxovirus-1, also known as Newcastle disease virus, which can be classified into three 

pathotypes based on their virulence in chickens. The World Organisation for Animal Health 

(OIE) defines Newcastle disease as infection caused by highly virulent strains of APMV-1 

viruses. This virulent form of New Castle disease (vND) is considered a foreign animal disease 

in the United States.

Clinical signs of vND vary and can include respiratory, neurological, reproductive, and intestinal 

signs. During this outbreak, clinical signs seen in chickens include loss of appetite, difficulty 

breathing, nasal discharge/ocular discharge, swelling around the eyes, diarrhea, blue combs, 

and death. Morbidity of unvaccinated chickens infected with vND virus strains can reach 100 

percent, and mortality ranges from 70 to100 percent. The severity of disease produced varies 

with the host species and the strain of the virus. Many other avian diseases present with clinical 

signs similar to vND; therefore, laboratory testing is necessary to distinguish between diseases. 

Newcastle disease is transmitted by inhalation or ingestion, and birds shed the virus in both 

feces and respiratory secretions. The virus can infect many species of domestic and wild birds. 

Chickens are highly susceptible, and other gallinaceous birds such as turkey, quail, and guinea 

are also susceptible. There are two species-adapted viruses that are genetically distinguishable 
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from those found in poultry in the absence of direct transmission: one is maintained in pigeons 

and doves, and another in double-crested cormorants (Brown and Bevins, 2017). Parrots have 

been reported to be infected with virulent viruses and have the potential to shed virus for long 

periods without shoeing clinical signs; however, data supporting virus maintenance in these 

species is lacking. A detailed summary of susceptible wild bird species is available in Appendix A.

Vaccination of commercial poultry against New Castle disease is common in the Americas, 

including the United States. The classical vaccine strains are distinguishable from other viruses 

by genome sequencing. Widespread vaccination of poultry was implemented in Mexico and 

several Central American countries in the early 2000s, and since this time divergence of 

subgenotypes circulating in vaccinated poultry has been documented (Susta et al., 2014; Garcia 

et al., 2013). Outbreaks of vND occurred in California, Nevada, and Arizona in 2002-2003 and in 

Texas in 2003. 

B. Description of Outbreak

May 16, 2018, to November 9, 2018

On May 16, 2018, the California Department of Food and Agriculture reported vND in sick 

backyard exhibition chickens presented to a veterinary clinic in Los Angeles County 

(Figure 1). The National Veterinary Services Laboratories (NVSL) confirmed vND virus on May 

17, 2018. This confirmation represented the first case of vND, (formerly referred to as exotic 

Newcastle disease) in the United States since 2003. Officials were first alerted to the possibility 

of a new finding of vND when an owner presented sick chickens to a California veterinary clinic. 

Biological samples were collected from the chickens and sent the California Animal Health and 

Food Safety (CAHFS) Laboratory where vND virus was detected. The CDFA responded to the 

incident by creating 3-km control areas around the premises associated with the index case and 

began targeted surveillance and outreach, including to feed stores and known exhibition bird 

premises. On May 24, 2018, NVSL confirmed vNDV in a backyard premises in San Bernardino 

County. On May 26, 2018, a USDA-APHIS incident management team joined the unified incident 

command in California. By this time, seven premises had been confirmed in San Bernardino 

County and two in Los Angeles County. On June 30, 2018, a premises in Riverside County was 

confirmed. On August 14, 2018, vNDV was confirmed in Ventura County. On September 25, 

2018, NVSL confirmed vNDV in a live bird market in Los Angeles County. From May 16 to 

November 9, 2018, 175 confirmed positive premises were identified in four California counties 

(Figure 2, Table 1).

The owner of the vND-infected live bird market in Los Angeles County reported first observing 

clinical signs approximately two weeks prior to presumptive diagnosis. Over the four weeks 

prior to reporting disease, the market received 43 shipments of live birds from four suppliers:

37 shipments of broilers, 4 shipments of spent hens, and 2 shipments of ducks. Bird shipment 

sizes ranged from 15 to 558 birds (mean=181 birds per shipment). Suppliers used dedicated 

cages that were washed and sanitized between shipments to transport birds. Suppliers typically 

made stops at more than one live-bird market on their routes. The market was visited by one 
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renderer, typically three times per week. The owner of the market reported rarely receiving 

birds from the community and no community birds were received in the 60 days prior to the 

onset of clinical signs. Active surveillance of other live bird markets in the area yielded no 

additional infected markets.

C. References

Brown V.R., Bevins S.N., A review of virulent Newcastle disease viruses in the United States and 

the role of wild birds in viral persistence and spread. Vet Res. 2017; 48: 68. 

Garcia S.C., Lopez R.N., Morales R., Olvera M. A., Marquez M. A., Merino R., Miller P. J., Afonso C.L., 

Molecular epidemiology of Newcastle disease in Mexico and the potential spillover of 

viruses from poultry into wild bird species. Appl. and Environ. Microbio. 2013; 79:4985-

4992.

Susta L., Hamal K.R., Miller P.J., Cardenas-Garcia S., Brown C.C., Pedersen J.C., Gongora V., Afonso 

C.L., Separate evolution of virulent Newcastle disease viruses from Mexico and Central 

America. J Clin Micro. 2014; 52:1382-1390.
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Figure 1. Counties with confirmed findings of vND from May 17 to November 9, 2018
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Table 1. Number of vND confirmed positive premises, by California counties and dates of earliest 

confirmation in each county, as of November 9, 2018.

County Confirmed Premises Earliest Confirmation Date in County

Los Angeles 38 17 May 2018

Riverside 32 30 June 2018

San Bernardino 104 24 May 2018

Ventura 1 14 August 2018

Total 175

Figure 2. California vND weekly case detection curve based upon the date the case definition2 was met for a 

presumptive positive flock, by day from May 17 to November 9, 2018.

 

2 Case definitions 

• Suspect case: domesticated bird or flock having clinical signs compatible with vND; or detection of APMV-1 

by rRT-PCR; or epidemiological information indicating exposure to vNDV

• Presumptive positive case: a suspect case with detection vNDV by the fusion-target rRT-PCR test at a 

laboratory designated by the Secretary of Agriculture

• Confirmed positive case: domesticated bird or flock from which vNDV has been identified at the NVSL as 

presumptive positive with confirmation of multiple basic amino acids (either directly via protein or by 

deduction through sequencing) in the fusion gene at the C-terminus of the F2 protein and phenylalanine at 

residue 117, which is the N-terminus of the F-1 protein. The term ’multiple basic amino acids’ refers to at 

least three arginine or lysine residues between residues 113 and 116; and/or the vNDV has an intracerebral 

pathogenicity index (ICPI) in day-old chicks (Gallus gallus) of 0.7 or greater.
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II. PHYLOGENETIC ANALYSIS AND DIAGNOSTICS

A. Virulent Newcastle Disease Virus

This section describes viruses characterized from the 2018 vND events in California (CA2018). 

The index case is chicken/California/18-016505-1/2018, which has an amino acid cleavage site 

of PGGRRQKR/FVGAII. The intracerebral pathogenicity index (ICPI) conducted on selected 

isolates in accordance with OIE guidelines confirms virulence and ranges from 1.67-1.753.

Chickens have been predominantly affected; other species from which the virus has been 

recovered include turkey, peafowl (peacock), duck, goose, and pigeon. Preliminary studies with 

the CA2018 index virus at the Southeast Poultry Research Laboratory suggest that it is highly 

chicken adapted and very infectious for chickens. 

Methods

Genetic sequence data from the virus is used to determine the cleavage site, which serves as 

disease confirmation. Additionally, full genomic sequence data are generated and analyzed to 

monitor virus evolution and to inform epidemiologic investigations. Genetic data are also used 

to confirm that diagnostic assays are fit for purpose. 

Results

The CA2018 virus (genotype Vb) is related to older Mexican-lineage viruses from Central 

American village poultry (Belize 2008, Honduras 2007), and the U.S. (smuggled parrot 1996, 

CA2002), which represent viruses from birds with low or no vaccine coverage. Preliminary 

genetic analysis of CA2018 virus isolates supports a single introduction followed by secondary 

spread based upon the high identity among available sequences from 41 chickens and 1 each 

from duck, goose, peafowl, pigeon, and turkey, representing 41 premises. Lack of epidemiologic 

data regarding the index premises and of contemporary sequence data (the most recent 

available related sequences are from 2008) contribute to the uncertainty surrounding the 

origin of the outbreak. Evolutionary analysis of available sequences with the CA2018 and 

CA2002 viruses suggest ongoing circulation of the virus; however, where and in what 

population remains unclear.

NOTE: The outcomes of phylogenetic analysis should be interpreted in context of all 

available virus and epidemiologic information and should not be used directly to infer 

transmission.

 

3 The World Organisation for Animal Health (OIE) defines Newcastle disease as an infection with a virulent APMV-1 

virus (vNDV) characterized by either an intracerebral pathogenicity index (ICPI) of 0.7 or greater in day-old chickens, 

or the presence of multiple basic amino acids at positions 113- 116 of the C-terminus of the fusion (F2) protein 

(either arginine (R) or lysine (K)), plus phenylalanine (F) at residue 117 of the F1 protein.
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B. Comparison to Other Viruses/Lineages 

The CA2018 virus is not related to classic Newcastle disease vaccine strains, nor to available 

strains from vaccinated poultry in Mexico (2000-2010). The virus is also unrelated to the 

species-adapted virus from columbids (pigeons, doves), and is not closely related to virulent 

viruses endemic to double-crested cormorants.

C. Diagnostics

Testing avian swabs/tissues for APMV-1 involves screening assays (real time reverse 

transcription polymerase chain reaction [rRT-PCR]), virus isolation, and characterization of the 

virus (sequencing and in vivo tests). The National Animal Health Laboratory Network (NAHLN) 

tests swab/tissue first by the APMV-1 matrix-target rRT-PCR test,

4 best suited to detect Class II 

viruses that contain low and highly virulent pathotypes, including vaccine viruses. Detections 

by the matrix-target test are subsequently tested by a fusion gene-target rRT-PCR test, which is 

designed to allow rapid identification of virulent viruses reportable in poultry. This approach

does not provide the genetic or geographic lineage of the virus. Sequence analysis of the virus 

compared to the assay primers and probes confirmed high identity between the CA2018 virus 

sequences and the fusion gene-target rRT-PCR test. A negative fusion gene-target test in the 

face of clinical signs requires further testing including virus recovery, sequence, and/or ICPI 

testing.

Under normal surveillance, all poultry samples with a nonnegative test result by APMV-1 PCR 

or virus isolation are forwarded to NVSL for confirmatory testing; for the current CA vND event, 

the NAHLN laboratory is using the highly matched fusion-target assay. The NVSL uses Sanger 

sequencing protocols to generate partial fusion gene sequence directly from the sample for 

virulence determination, where sufficient viral RNA is present. Whole genome sequencing is 

conducted on all isolated viruses, and select viruses are further characterized by ICPI in specific 

pathogen-free chickens. 

The NVSL confirms the virus lineage and virulence through molecular sequencing. Where no 

virus can be recovered nor sequence obtained directly from sample(s), the virulence is 

determined by the clinical presentation of the flock compared to the USDA vNDV case 

definition.

 

4 PCR results from the NVSL are reported as “detected” or “not detected” and include the cycle threshold (Ct) value. 

The lower the Ct value, the more viral nucleic acid was detected.
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III. POPULATION AT RISK

A. Predicting Areas of Backyard Bird Ownership

The distribution of backyard bird flocks in the United States is currently unknown. Statistical 

modeling, however, can be used to estimate the likely locations and densities of backyard flocks 

in a given geographic area using socioeconomic and demographic variables that historically 

have been shown to be related to bird ownership. This approach was used to develop 

neighborhood-level spatial data to facilitate the creation of risk maps to identify and prioritize 

areas for surveillance during the 2002-2003 outbreak of vND in the United States (Freier et al., 

2004, Freier et al., 2007). Building on that historical work, we aimed to identify areas with 

increased probability of backyard poultry ownership to inform surveillance response efforts for 

the current outbreak.

Methods

A Bayesian hierarchical model for spatial areal unit data was used to analyze socioeconomic 

and demographic variables that have previously been found to be associated with urban poultry 

ownership. Census block groups were used as the unit of analysis. The number of backyard 

flocks identified during the previous vND outbreak in 2002-2003 were tallied for each census 

block group. During the 2002-2003 outbreak, all homes within 1 mile of an affected premises 

were queried about backyard poultry ownership, resulting in a near census of backyard poultry 

ownership for some census block groups. Census block groups within 1 mile of affected 

premises were then assumed to have all flocks identified, and the total number of households 

reported in the census data was used as the total sample size. These census block groups were 

then used as data to fit the Bayesian model. 

The Bayesian model used a binomial likelihood conditional on historical sociodemographic and 

economic risk factors (Figure 3). The model included a spatial random effect using a 

convolution model that allows for both weak and strong spatial autocorrelation with 

neighboring census block groups. Prior to model fitting, 5 percent of the data was randomly 

withheld for out-of-sample model validation. The withheld data was identified using conditional 

Latin hypercube sampling. Models were fit using JAGS in R.

Results

Human population size, home value, education level, housing density, household income, and 

household size were all significant predictors. Figure 3 presents the predicted coefficients for 

the sociodemographic and economic predictors used in the model. Based on the model’s 

predictions, a single surveillance zone (10km) could have almost 4,000 premises with backyard 

poultry, while the greater Los Angeles area is predicted to have over 11,000 backyard poultry 

premises. Figure 4 presents the predicted distribution of backyard flocks by census block group 

within the current surveillance zone and control areas.
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Generally, the model performs well, explaining 79.9 percent of the deviance in the spatial 

distribution of backyard poultry ownership during the 2002-2003 outbreak. Comparison of the 

predicted number of households with backyard poultry with the out-of-sample data found a 

Pearson’s correlation of 0.67, indicating that the model has good predictive capacity.

Figure 3. Preliminary sociodemographic variables used in the model to predict the presence of backyard poultry 

in a census block in Southern California.
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Figure 4. Predicted number of backyard flocks, by census block group in Southern California.

Summary

The outbreak area is likely to have a very dense population of backyard poultry. In addition, 

there was a strong spatial pattern to the distribution of backyard poultry ownership, indicating 

that a spatially targeted approach might improve surveillance efficiency. Work continues on 

improving the model and incorporating data and predictors related to the probability that vND 

is present in the census block groups. Formal model selection has not been implemented yet but 

might improve the predictive abilities of the model. The data used to fit the model was 

restricted to block groups in Southern California to facilitate model fitting (i.e. limit cpu time).

However, data are available throughout California, Nevada, and Arizona for the 2002-2003 vND 

outbreak. Including these data in the model might improve prediction and applicability to other 

regions of the United States.
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B. References

Freier, J. E., et al. "Spatially-targeted surveillance for Newcastle disease in southern California." 

Proc. GISVET 4 (2004): 3-5.

Freier, J. E, Miller, R. S, & Geter, K. D. (2007). Geospatial analysis and modelling in the 

prevention and control of animal diseases in the United States. Veterinaria italiana, 43(3), 

549-557.
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IV. EPIDEMIOLOGIC RISK

A. Case Control Study

A case-control epidemiological analysis was performed on confirmed and presumptive 

positive virulent Newcastle disease (vND) backyard premises, dangerous contact premises, 

and noninfected premises. Data were obtained from in-person interviews using the CDFA 

Non-Commercial Premises Virulent Newcastle disease Epidemology Questionnaire; 

questionnaire data were entered into the USDA’s Emergency Management Response System

(EMRS).

Methods

Data were analyzed for 912 premises: 137 confirmed or presumptive positive premises, 68

dangerous contact premises, and 712 noninfected premises. The analysis included data from 

questionnaires that were completed from May 16 to November 9, 2018, and includes all 

confirmed and presumptive premises for which questionnaire data were entered into the EMRS 

as of November 9, 2018. The questionnaire form was updated in July 2018 with additional 

questions; 69 respondents completed the original questionnaire and 848 completed the 

updated questionnaire. Questionnaires were not complete for all premises, such as in cases in 

which the owners refused to provide answers to certain questions. The number (n) of 

responding premises is noted in Table 1. Odds ratios, p-values and 95-percent confidence 

intervals for flock characteristics and other risk factors were estimated by univariate logistic 

regression, using confirmed/presumptive premises as cases and noninfected premises as 

controls. Dangerous contacts were excluded from the regression analysis. To identify significant 

risk factors, while controlling for possible confounding variables, two multivariable logistic 

regression analyses were performed. The first included questions found in both versions of the 

questionnaire, while the second included questions found only in the newer version of the 

questionnaire. All variables that had a significant (p-values < 0.1) predictive effect on being a 

case were included in the analysis, and backward stepwise elimination was used to obtain final 

models.

Results

Premises characteristics 

The reported flock sizes ranged from 1 to 853 birds (mean=51, median=18 birds). Thirty-three

percent of all backyard flock owners had multiple bird species on their premises. These 

premises primarily had backyard chickens (82.9 percent). Fewer premises had exhibition 

birds/game fowl (8.5 percent), and ducks/geese (11.5 percent). Other types of birds were 

reported on 30 percent of premises; the most commonly reported species were pigeons, 

turkeys, peafowl, parrots, and cockatiels. Besides birds, 35 percent of owners had other 

livestock species on their backyard premises, 76 percent had dogs/cats, and 8.5 percent 

reported other non-bird species.
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Housing types 

The majority of respondents (59.6 percent) reported housing birds outdoors in cages or coops, 

with 28 percent housing birds outdoors in open top pens or enclosures, and only 7 percent 

housing birds indoors. Only 5 percent of respondents reported keeping birds individually 

tethered, and 35 percent reported having free-range birds.

Illness and mortality 

Sixty-four percent of case premises reported bird illness, and 65.7 percent reported mortality. 

The mean time reported between onset of illness and presumptive detection was 9.6 days 

(median=6.0, range 1-90 days). The mean time between onset of mortality and presumptive 

detection was 10.1 days (median=4.7 days, range 1 to 90 days). As an indicator of background 

morbidity and mortality, the percentage of control premises reporting illness was 10.5 percent 

and mortality was 17.6 percent. 

Risk factors – Univariate analysis 

• Flock size

Case premises reported larger flock sizes than control premises. The odds of being a 

case were significantly greater for flock sizes greater than 100 birds (OR = 11.6, 

95% CI: 6.4-21.0) or from 20 to 99 birds (OR = 5.0, 95% CI: 3.0 – 8.5) when 

compared with flock sizes of fewer than 20 birds.

• Bird types

Case premises were more likely to report having flocks that included exhibition birds or 

other non-chicken bird species than control premises (OR = 7.7, 95% CI: 4.6-12.8). 

The odds of becoming a case premises were also higher when roosters comprised more 

than 50% of the adult birds in the flock (OR=4.3, 95% CI: 2.7-6.7).

• Contact with other domestic and wild birds 

Although only 8.8 percent of case premises reported keeping birds at other premises, 

the odds of being a case were higher (OR = 3.9, 95% CI: 1.8-8.2) when birds were 

kept at multiple locations.

A high percentage of both case and control premises reported having neighbors with 

birds (75.9 percent and 55.0 percent, respectively); however, premises that 

reported that their birds visit neighboring properties or that their neighbors’ birds 

visit their property did not have increased odds of becoming a case. Contact with 

wild birds (OR = 3.5, 95% CI 2.0-6.2) was associated with greater odds of becoming 

a case premises.

• The use of Newcastle vaccine

The percentage of premises reporting the use of Newcastle vaccine was low overall (6.9 

percent). The percentage of case premises that reported using Newcastle vaccine 

was much higher than the percentage of controls that reported using Newcastle 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

USDA APHIS VS 17

vaccine (18.9 percent vs 5.6 percent, respectively), and the risk of disease was 

greater among flocks that reported use of Newcastle vaccine (OR = 4.2, 95% CI: 2.4-

7.5).

Risk factors – Multivariate analysis 

Many of the risk factors described previously are related. A multivariate analysis was 

performed in order to provide adjusted odds ratios for risk factors, while taking into account 

the interrelationships among these flock management characteristics and behaviors. For the 

multivariate analysis including both versions of the questionnaire, 103 cases and 579 controls 

were examined. Larger flock sizes (OR = 3.9, 95% CI: 2.2-7.1 for 20-99 birds; and OR = 5.7 95% 

CI: 2.8-11.7 for flocks with more than 99 birds), the presence of game fowl on the premises (OR 

= 4.6, 95% CI 2.5-8.6), and having greater than 50 percent of adult birds as roosters (OR = 2.4, 

95% CI 1.4-4.1) significantly increased the odds of becoming infected. A nested analysis looking 

only at questions found in the newer version of the survey (84 cases and 622 controls) 

identified these same factors, as well as wild bird contact with domestic birds (OR = 3.1, 95% 

CI: 1.7-5.9), and having neighbors with birds (OR = 2.2, 95% CI: 1.2-3.9) as significant risk 

factors. 

Summary

These results suggest that flock size, ownership of exhibition birds, a high proportion of 

roosters in the flock, and housing that facilitates contact with nearby domestic and wild birds 

are risk factors for vND infection in this population. Some of these practices have been shown to 

be risk factors in other studies or previous vND outbreaks in the United States, as summarized 

below. However, not all epidemiology questionnaires were complete, and it is likely there is 

misclassification bias for some of these results, such as the type and number of birds on 

premises, the number of owners, and use of Newcastle vaccine; therefore, results should be 

interpreted with caution. 

Summary of Historical Epidemiologic Risk Factors

An epidemiological study of backyard premises during the 2002-2003 California vND outbreak 

identified the following risk factors for vND infection on premises: presence of game fowl, 

presence of feral chickens, flock sizes larger than 40 birds, and multiple owners of a flock. 

Epidemiological descriptions of infected backyard premises in the 1971-1974 outbreak 

identified contact with infected commercial layer farms as the primary source of infection, 

followed by active trading of birds among backyard flocks and purchases of infected exotic 

birds from dealers. The severity of infection among commercial premises during the 1971-1974 

outbreak was attributed to the high density of egg-laying premises and extensive contact among 

those premises. In both the 1971-1974 and 2002-2003 California vND outbreaks, a suspected 

risk factor for vND infection in commercial premises was movement of contaminated 

equipment, such as egg carts.
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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B. References

Bulaga, L.L., Dargatz, D., Garber, L., Miller, R., Rhoads, G. (2004). Risk Factors for exotic 

Newcastle disease infection in non-commercial flocks, California. Unpublished Report

Burridge, M. J., Riemann, H. P., Utterback, W.W. (1975). Methods of spread of velogenic 

viscerotropic Newcastle disease virus in southern California. Avian Diseases, 19(4), 666-

678.

Speers, R., Webb, M., Grund, M, Howell, B., Hughes, C., Myrus, E., and, Silverman, J. (2004).

Reconstruction of response operations to eradicate exotic Newcastle disease in 2002-2003. 

Technical report produced for USDA by the CNA Corporation. 

USDA-APHIS (1978). Eradication of exotic Newcastle disease in southern California 1971-1974. 

APHIS-91-34 Technical Report

Utterback WW, Schwartz, J.H. (1973) Epizootiology of velogenic viscerotropic Newcastle 

disease in southern California, 1971-1973. J Am Vet Med Assoc. 163(9), 1080-1088.
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Table 2. Characteristics of backyard case premises (confirmed/presumptive positive for vND),

dangerous contact (DC) premises, control premises (C), and odds ratios (OR) and p-values 

calculated by univariate logistic regression (dangerous contacts excluded).

Table 3. Adjusted odds ratios (OR) for significant risk factors identified in multivariate regression analyses. 

Characteristic Level OR p-value

Number of birds1 1-19 Ref

20-99 5.4 0.04

100+ 9.0 0.001

Game fowl on premises1 4.8 0.001

Adult birds >50% roosters1 2.4 <0.001

Neighbors have birds2 2.2 0.007

Wild birds have contact with domestic birds2 2.6 0.003 1Results from analysis that included questions found on both versions of the survey (102 cases and 538 controls)

2Results from analysis that included questions found only on the newer version of the survey (84 cases and 604 controls)

Characteristic Level

n

Case DC Control OR p-value

Number of birds 1-19 20/125 8/43 385/698 Ref

20-99 64/125 9/43 245/698 5.0 <0.001

100+ 41/125 8/43 68/698 11.6 <0.001

Bird species on premises Backyard chickens 98/125 23/25 635/695 0.34 0.207

Exhibition birds 37/125 5/25 36/695 7.7 0.005

Ducks/geese 18/125 5/25 82/695 1.3 0.329

Other species 45/125 8/25 217/695 1.2 0.019

Adult birds >50% roosters 43/103 3/23 84/583 4.3 0.001

Owners keep birds on other 

premises

12/119 1/23 19/676 3.9 <0.001

Nonbird species or wildlife on 

premises

17/67 3/9 38/193 1.4 0.328

Housing Inside home

Outdoor open top

Outdoor cage/coop

Individual tether

Free range

3/93

34/93

76/93

6/93

46/93

0/19

10/19

13/19

0/19

10/19

52/671

217/671

458/671

43/671

266/671

0.4

1.2

2.1

1.0

1.5

0.126

0.417

0.009

NA

0.072

Movement of new birds onto the premises within 30 

days prior to the interview 12/121 2/24 44/674 1.7 0.123

Movement of birds off the premises within 30 days prior 

to the interview 8/119 0/23 30/651 1.5 0.33

Give/sell eggs 11/92 2/19 79/662 1.0 NA

Neighbors have birds 66/87 13/19 357/649 2.6 <0.001

Birds visit neighbors 16/88 2/18 392/624 1.8 0.063

Wild birds have contact with domestic birds 77/92 16/20 392/660 3.5 <0.001

Newcastle disease vaccine No 84/122 22/25 589/683 Ref

Yes 23/122 2/25 38/683 4.2 <0.001

Unsure 15/122 1/25 56/683 1.9 0.776
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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V. ESTIMATING DISEASE SPREAD

A. Flock Disease Spread Model–Early Outbreak

Methods

An epidemiologic scenario was developed in InterSpread Plus® v. 6.01.44 (Stevenson et al., 

2013) to model the introduction and spread of vND from confirmed premises in San Bernardino 

county, California following the first case detection. Commercial and backyard farm units from 

the Western United States (17 states) were incorporated into the model. The Farm Location and 

Animal Population Simulator was used to generate likely farm locations based on geospatial 

characteristics, with backyard farm locations adapted from current and historic outbreakrelated data. Model parameters were developed to reflect the impact of sustained outreach 

activities, incorporate preliminary experimental data on viral pathogenesis in chickens, include 

current strategies for active surveillance of commercial operations, and describe the potential 

geographic extent of disease spread during the silent-spread period. The model was updated 

regularly in order to provide timely results to the response during the early phase of the 

outbreak.

Results

Summary outcomes for a 300-iteration scenario were generated from ten seeded-sites. The 

seeded sites were based on the latitude/longitude of initial confirmed cases of vND in San 

Bernardino County. Simulations include control activities implemented in the vND response, 

including outreach, quarantine, euthanasia/depopulation of detected premises, movement 

controls, tracing, and active and passive surveillance. Note: These outcomes are based on a 

completely naïve poultry population. The variable levels of vaccination applied within backyard 

farms is not explicitly modeled in this scenario.

The summary of results and their potential implications for the current vND outbreak are as 

follows:

• Initial disease spread commonly involves direct movements of infectious birds, but local 

spread becomes more prevalent as outbreaks become greater than 50 infected 

premises.

Direct contacts associated with live animal movements accounted for 36 percent of 

spread for simulated outbreaks that resulted in less than 50 infected premises, and 

27 percent of spread for simulated outbreaks that resulted in 50 or more infected 

premises. 

Local area spread became more prevalent as simulated outbreaks became larger, being 

responsible for 56 percent of disease spread for simulated outbreaks that resulted 

in 50 or more infected premises.

Implications: As the number of detected premises continues to increase, outcomes 

from simulated outbreaks suggest that local spread of vND among premises 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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might be responsible for additional infections. Local spread is associated with 

distance between infectious and susceptible premises and represents 

mechanisms of spread that are difficult to trace, such as movement of free 

ranging birds, wildlife, or fence-line contact. Good biosecurity practices and 

measures are the best way to prevent local spread (e.g., keeping outside birds in 

cages, moving cages away from neighboring fence lines, repairing 

damaged/missing fences, rodent control, covering/tarping cages to decrease 

wildlife/rodent/loose-chicken exposures, and the spread of vND viral particles

into the environment).

• Small backyard operations5 are the primary premises involved in outbreaks; large 

backyard operations or commercial poultry farms have a lower likelihood of becoming 

infected.

Across all simulated outbreaks, large backyard operations represented slightly less than 

1 percent of all infected premises, and commercial poultry farms represented 0.14 

percent of all infected premises. 

vND-infected small backyard operations in 100 percent of all simulated outbreaks, large 

backyard operations in 22 percent of all simulated outbreaks, and commercial 

poultry farms in 7 percent of simulated outbreaks. 

All spread to commercial operations resulted from indirect contact (e.g., people or 

vehicles moving from operation to operation) with infected, primarily small 

backyard operations. 

Implications: Unless generated by indirect contacts with infected backyard 

operations, outcomes from simulated outbreaks suggest a low probability of 

spreading vND to commercial farms. 

• The extent of spread for simulated outbreaks is primarily in San Bernardino, Los 

Angeles, and Riverside counties. 

When considering disease spread within the silent period of the outbreak (three days 

prior to the first observation of clinical signs to the day of first detection), spread 

from infected premises in San Bernardino County to backyard chicken premises in 

Riverside County occurred in 66 percent of simulated outbreaks and to backyard 

chicken premises in Los Angeles County in 65 percent of simulated outbreaks.

 

5 In the model operations are defined as follows:

1) Commercial poultry farms: more than 75,000 table egg laying chickens, or more than 100,000 meat-type 

chickens, or more than 30,000 meat-type turkeys

2) Large backyard operations: more than 1,000 birds but fewer than the number of birds described for 

commercial operations

3) Small backyard operations: fewer than 1,000 birds
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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In the current modeling scenario, 42 percent of simulated outbreaks involved 50 or 

more infected premises, and 19 percent of outbreaks involved 100 or more infected 

premises. 

Implications: Simulated outbreaks suggest future detections in other Southern 

California counties, most commonly Riverside and Los Angeles. In addition, 

some infected premises might not be detected due to natural viral elimination 

from these premises (i.e., birds die and go unreported) and/or no new, naïve 

birds being brought onto previously infected premises.

B. Comparing Alternative Control Strategies–Mid-Outbreak

Methods

As the outbreak progressed, selected parameters were revised from preliminary scenarios 

described previously to compare the impact of alternative control strategies on the severity and 

duration of simulated vND outbreaks. This analysis was performed and supplied to the Incident 

Coordination Group approximately 3 months into the outbreak.

Summary outcomes for a series of four 250-iteration scenarios were generated from 57

selected sites. The initially infected sites in the model were based on the latitude/longitude of 

initial confirmed cases of vND in San Bernardino and Riverside counties and from premises 

frequently infected during the silent spread period identified in previous modeling analyses. 

Each simulated outbreak was allowed to run for a maximum of 365 days from detection of the 

first infected premises. All disease spread was considered lateral spread between infected and 

susceptible farms. Simulations varied in the availability of resources for conducting disease 

control activities, including outreach, quarantine, euthanasia/depopulation of detected 

premises, movement controls, tracing, and active surveillance. We assumed a completely naïve 

population, and the variable levels of vaccination possibly applied within backyard farms was

not modeled explicitly.

Four levels of response were evaluated. A specific combination of integrated control strategies 

was associated with each response option, with a general increase in response intensity from 

response 1 to 4 (see Appendix B for detailed information on specific activities modeled in each 

response option). Disease control activities are identical for the first 161 days of each scenario 

to reflect the actual outbreak response up to that point in time. Alternative disease control 

activities, based on resource level, were applied at day 162 of each iteration (75 days post first 

detection). The model was run for 250-iterations for each of the four response options. 

A summary comparison of response options and associated control activities are described in 

Table 4. The ‘X’s are provided to estimate a qualitative comparison between control strategies. 

They are not intended to approximate a quantitative comparison between response options. 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Table 4. Qualitative summary comparison of the four alternative response options examined.

Response 

Option

Surveillance 

Capacity

Surveillance 

Zones

Movement 

Restrictions

Depopulation 

Capacity

Depopulation 

Zones

1 X X X X X

2 XX X XX XX X

3 XX XX XX XX XX

4 XXX XX XXX XXX XXX

Results

The best response option was dependent on the desired outcome. If limiting disease spread, as 

expressed by the mean number of infected backyard premises, was the only goal, response 

option 4 achieved the greatest reduction in the number of infected backyard premises. 

Reducing the total number of infected commercial premises was best achieved with response 

options 3 or 4.

Both response options 3 and 4 reduced the likelihood of extremely large outbreaks. However, 

any increase over response option 1 reduced the mean number of infected backyard premises, 

suggesting that minimal response is unlikely to achieve an adequate reduction in disease spread 

among backyard premises.

Outbreak duration, as expressed by the percentage of simulated outbreaks continuing into the 

months following the application of the response option was shortest, on average, for response 

options 3 or 4. Pronounced differences were observed when comparing response options 1 or 2 

with response options 3 or 4, with little difference observed between response options 3 and 4. 

Surveillance effectiveness, as expressed by the percentage of infected premises that were

detected through passive and active surveillance activities, was significantly improved under 

response option 4, in comparison with any of the other response options. Little difference was

observed in detection rates between response options 1 and 2, with some improvement 

observed with response option 3. 

With all response options, the predominant site of disease spread shifts from San Bernardino 

County to Los Angeles County shortly after applying the alternative response. This shift was

most pronounced with response options 3 or 4. Surveillance surges within the first 30 days 

post-implementation of the response option generally increased the rates of detection in San 

Bernardino County and reduced further spread within the county, to the extent that the 

majority of future infections occurred in Los Angeles County.

The largest and longest simulated outbreaks frequently involve significant disease spread 

within Los Angeles County, irrespective of the selected response option. A relatively small 

number of simulated outbreaks became extremely large (greater than 1,000 infections) and 

persistent (remaining active for at least 3 months following the selection of a response 

strategy), irrespective of the selected response option.
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C. Within-Flock Transmission Model

Within-flock models and their results are used to evaluate surveillance options, support risk 

assessments, and assess different control measures. Statistical distributions for bird-level 

disease state durations are key inputs for within-flock disease transmission models. 

We estimated bird-level disease state durations and a lower bound on the rate of transmission 

(β) using experimental data available from the peer-reviewed literature and unpublished data 

provided by the USDA, ARS, Southeast Poultry Research Laboratory (SEPRL). The estimated 

parameters were then used to predict the time to detect vND in an unvaccinated, 50-bird, 

backyard flock, based on observation of increased mortality (2 or more dead birds within a 3-

day period).

Methods

Estimating the latent and infectious periods and time to death at the bird level 

For this analysis, we defined the latent period as the interval between when an individual bird 

is exposed to the virus and when it begins shedding virus in detectable concentrations. We 

estimated the distribution of the latent period from viral shedding data collected on various 

days post inoculation (DPI), as reported in experimental studies in the literature and from 

unpublished SEPRL data6. Data were available from 122 unvaccinated chickens. Oropharyngeal 

swabs were collected at specific sampling times post inoculation and starting on 1 or 2 DPI. 

These data points represented the CA 2018 vND strain, CA 2002-2003 vND strain, and a 

mesogenic vND strain. An additional 73 data points (birds) were available for the time to death 

post inoculation (observed at daily intervals). These data included unvaccinated chickens 

inoculated with vND-CA 2002-2003, vND-CA 2018,vND-Peru 2008 or vND-India 2012. Contact 

bird data from unvaccinated birds were not included for estimating the infectious period, as 

data was only available for five birds, and the first sampling time was 2 days post contact. The 

non-inoculated birds in this experiment all died by day 6 post contact, indicating that the range 

of time to death is comparable to that for inoculated birds.

The infectious period was defined as the interval from when an individual bird begins shedding 

virus in detectable quantities to when it either recovers or dies. In several experimental studies, 

only the time to death was observed, and oropharyngeal swabs were not collected. We jointly fit 

the parameters of the latent and infectious periods given all of the observed data, including 

instances in which only the time to death was observed.

We used the Metropolis MCMC algorithm implemented in R for parameter estimation. The three 

chains were run for 10,000 iterations with burn-in of 2,000 iterations. There was no significant 

autocorrelation beyond 60 lags. Uniform priors with wide limits that included the MLE estimate 

were used in the current analysis. 

 

6 Courtesy of Kiril M. Dimitrov, Helena L. Ferreira, Mary Pantin-Jackwood, Tonya L. Taylor, Iryna V. Goraichuk, 

Claudio L. Afonso, David L. Suarez
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Estimating the rate of transmission (β) 

The adequate contact rate is a key parameter that determines the rate of within-flock spread. In 

the SEIR model, the adequate contact rate or the transmission parameter (β) is the average 

number of contacts that a bird has with other birds per unit time, such that the contact can 

transmit infection. We estimated the adequate contact rate using data provided in Miller et al., 

2003, in which the transmission to contact birds was studied. We used direct forward 

simulation to obtain the posterior distribution for the adequate contact rate, given the observed 

experimental data on viral shedding and the timing of death for the contact birds.

Estimating Time to Detection in Unvaccinated Flocks 

We estimated the time to detection in backyard flocks of 50 unvaccinated birds using a 

stochastic within-flock simulation model (SEIR), applying the maximum likelihood estimates of 

the parameters (as described above) and a trigger criteria of observing 2 or more dead birds 

within a 3-day period.

Results

Latent and infectious periods and time to death at the bird level 

• Latent Period Parameters

The mean latent period was 0.40 days (95% CI: 0.30 – 0.51 days).

• Infectious Period

The mean infectious period was 4.33 days (95% CI: 4.03-4.98 days). The maximum 

likelihood estimate for the infectious period was shape parameter of 13.07 (95% CI:

3.6-18.6) and a scale of 0.33196 (95 % CI: 0.23-1.33).

• Time to Death

The maximum likelihood estimate for the bird-level mean time to death was 4.73 days 

(95% CI: 4.45-5.4 days).

Rate of transmission (β) 

There was considerable uncertainty for this parameter, given the limited amount of data 

available. However, based on the estimated posterior, a value of 1.7 contacts per day (95% CI: 

1.69-9.79 adequate contacts per day) may be used as a conservative estimate.

Time to detection in unvaccinated flocks 

Under the baseline scenario, the time to detection was 5.5 days (95% PI: 4-7 days) based on 

20,000 iterations of the model.

D. Spatial and Spatiotemporal Patterns of the Outbreak

Knowledge of disease patterns in space and time can identify areas at higher risk for disease 

spread and allow disease control, prevention, and surveillance strategies to be implemented 

effectively (Ward, 2007). We performed a spatiotemporal analysis on confirmed and 

presumptive positive vND in backyard premises in California. We obtained data on confirmed 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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and presumptive positive premises from in-person interviews using the CDFA Non-Commercial 

Premises Virulent Newcastle Disease Epidemiology questionnaire, which were entered into the 

USDA’s Emergency Management Response System (EMRS). For population data, we used the 

results of a spatial analysis predicting the geographic area and density of backyard bird 

ownership in California at the census block level (see Section III, Part A: Predicting Areas of 

Backyard Bird Ownership).

Methods

We used spatial and spatiotemporal scan statistics to detect significant high-risk clustering of 

vND cases (Kulldorff, 1997). For the analysis, we defined cases as confirmed or presumptive 

positive premises. Data from 137 cases detected from May 16 to August 25, 2018, in Los 

Angeles, Riverside, San Bernardino, and Ventura counties, were included in the analysis. Case 

information consisted of the location and reported date of onset of clinical disease. Cases were 

aggregated at the census block level within each county. Population information consisted of 

the estimated number of premises of predicted backyard bird ownership in each census block 

for the outbreak area. We used the centroid (latitude, longitude) of each census block as

location information for the analysis. 

A Poisson model was used to estimate the number of cases that might be expected to occur in 

the absence of any clustering. For both the spatial and spatiotemporal cluster analyses, data 

were scanned with a 5-km radius spatial window. For the spatiotemporal cluster analysis, a 

temporal window of 15 days was used, which is the higher range of the flock-level incubation 

period of vND. We determined statistical significance (p-value < 0.05) of clusters using the 

likelihood ratio test and Monte Carlo simulation implemented in SaTScan (version 9.6).

Results

The 137 detected cases from May 16 to August 25, 2018, were located within 14 control areas. 

The cases were located within 31 census blocks, with the number of cases ranging from 1 to 26 

premises within an individual block. San Bernardino County had the highest number of 

reported cases, with 91 of the 137 detected cases occurring in this area (Figure 5). 

The primary (or, most likely) spatial and spatiotemporal statistically significant clusters (log 

likelihood ratio = 264.92 and 114.01, respectively; p-value <0.001 for both) of detected vND 

cases occurred in the same control area in San Bernardino County (Figure 6). In the spatial 

cluster, 75 cases were reported out of an estimated at-risk population of 222 premises with 

predicted backyard bird ownership (or, 34 cases per 100 premises at risk). Based on the 

Poisson model, 1.15 cases would be expected to be detected from this population size; 

therefore, 65.2 times as many cases were observed as would be expected to be reported in this 

area. In the primary spatiotemporal cluster, 26 cases occurred out of an estimated population 

at risk of 174 premises from June 26 to July 10, 2018 (Figure 7). In this cluster, the relative risk 

of cases occurring in this area and time period was 242.20 times more likely, relative to outside 

this area (Table 5). 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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An additional three secondary statistically significant spatial and spatiotemporal clusters 

(p-value < 0.01) were identified within three control areas in Los Angeles, Riverside, and San 

Bernardino counties (Figure 6). The number of cases within the spatial clusters ranged from 10 

to 17 cases within an estimated population of 250 premises with backyard birds (or, 4 to 7 

cases per 100 premises at risk). The number of cases within the spatiotemporal clusters ranged 

from 5 to 10 cases within an estimated population of 237 premises with backyard birds, which 

occurred from May 23 to August 15, 2018 (Table 5 and Figure 7). The relative risk of cases 

occurring within these areas was highest in Los Angeles County, followed by Riverside and San 

Bernardino counties. 

Twenty-four of the 137 cases detected during the time period for this analysis did not occur in 

any spatial or spatiotemporal cluster. In addition, there was no clustering identified in 10 of the 

14 control areas. Only one case was detected in Ventura County; no areas of spatial or 

spatiotemporal clustering were identified in this county.

Figure 5. Cumulative number of confirmed and presumptive positive vND premises detected in California 

from May 16 to August 25, 2018; data are aggregated at the census block level.
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Figure 6. Location of spatial (upper) and spatiotemporal (lower) clusters of vND in California from May 16 to

August 25, 2018.
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Figure 7. Time period of occurrence of spatiotemporal clusters of vND in California from May 16 to August 

25, 2018. Colors shown relate to the lower part of Figure 6.

Table 5. Spatiotemporal clusters of vND cases in California from May 16 to August 25, 2018.a

No.: number; Exp.: expected

a

All clusters were statistically significant (p-value < 0.001) b

1, primary cluster; 2–4, secondary clusters c

Number of premises with predicted backyard bird ownership 

d

Radius is zero as there is only one census block in the cluster

Conclusions

Results identified specific geographic areas at the census block level within four vND control 

areas of significant spatial and spatiotemporal disease clustering. The primary spatial and 

spatiotemporal clusters were located within the same control area in San Bernardino County, 

identifying this area as the location of the highest occurrence of vND cases detected from May 

16 to August 25, 2018. This finding is consistent with the subsequent increase in outbreak 

response activities initiated in this area based on epidemiologic investigations during the end of 

the time period that this clustering occurred (June 26 – July 10, 2018). 

The spatiotemporal cluster identified in Los Angeles County had the highest relative risk of vND 

occurrence (RR: 25,544.73; Table 5). Although the clustering occurred over a two-day time 

period (July 13 – 14, 2018) within an area with low numbers of predicted backyard flocks, the 

results indicated that this area might have a high risk of vND spread. In the weeks following the 

Clusterb

Radius 

(km) Time Period

Estimated 

Populationc

No. Census 

Blocks

No. 

Cases

No. 

Exp.

Log 

Likelihood 

Ratio

Relative 

Risk

San Bernardino County

1 1.36 June 26 – July 10, 2018 174 6 26 0.13 114.01 242.20

4 2.13 May 23 – 30, 2018 158 7 7 0.06 26.08 114.87

Los Angeles County

2 0d July 13 – 14, 2018 2 1 5 0.0002 45.65 25,544.73

Riverside County

3 0.71 August 5 – 15, 2018 77 2 10 0.04 44.90 250.77
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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period included in this analysis, a sharp increase in the number of detected flocks occurred in 

this area, and enhanced disease detection and control activities were established. 

This approach has some limitations. vND cases may be underreported, which can result in 

misclassification of cases and non-cases. In addition, actual data of true backyard bird 

ownership in the outbreak area remains limited. We used an estimated population at risk 

based on the predicted number of premises of backyard ownership using 2002 census block 

data. As such, the true number of premises with backyard birds used in this analysis may be 

under- or overestimated, resulting in the number of detected clusters and estimated risk to be 

over or underestimated. 

Case detections have been ongoing in the California vND outbreak area after the time period of

this analysis. The addition of newly detected cases could further enhance and/or change the 

results presented here. Future analyses will incorporate additional cases and evaluate the 

spatial distribution of risk factors that might further explain areas at higher risk for vND 

occurrence. 

In conclusion, the identification of significant spatial and spatiotemporal clustering patterns of 

vND in California from May to August, 2018, support control strategies of targeting high risk 

areas for disease spread with increased response efforts in order to maximize the effectiveness 

of disease response strategies and control the outbreak.

E. References

Burdett, C. L., Kraus, B. R., Garza, S. J., Miller, R. S., & Bjork, K. E. (2015). Simulating the 

distribution of individual livestock farms and their populations in the United States: An 

example using domestic swine (Sus scrofa domesticus) farms. PloS One, 10(11). doi: 

10.1371/journal.pone.0140338

Costa-Hurtado M, Afonso CL, Miller PJ, Shepherd E, Cha RM, Smith D, Spackman E, Kapczynski 

DR, Suarez DL, Swayne DE: Previous infection with virulent strains of Newcastle disease 

virus reduces highly pathogenic avian influenza virus replication, disease, and mortality in 

chickens. Veterinary research 2015, 46(1):97

Desingu P, Singh S, Dhama K, Kumar OV, Malik Y, Singh R: Clinicopathological characterization 

of experimental infection in chickens with sub-genotype VIIi Newcastle disease virus 

isolated from peafowl. Microbial pathogenesis 2017, 105:8-12

Diel DG, Susta L, Cardenas Garcia S, Killian ML, Brown CC, Miller PJ, Afonso CL: Complete 

genome and clinicopathological characterization of a virulent Newcastle disease virus 

isolate from South America. J Clin Microbiol 2012, 50(2):378-387

Kapczynski DR, King DJ: Protection of chickens against overt clinical disease and determination 

of viral shedding following vaccination with commercially available Newcastle disease virus 

vaccines upon challenge with highly virulent virus from the California 2002-2003 exotic 

Newcastle disease outbreak. Vaccine 2005, 23(26):3424-3433
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Kulldorff M. (1997). A spatial scan statistic. Communications in Statistics: Theory and Methods, 

26:1481-1496.

Kulldorff M. and Information Management Services, Inc. (2009). SaTScanTM v9.6: Software for 

the spatial and space-time scan statistics. http://www.satscan.org/.

Miller PJ, Afonso CL, El Attrache J, Dorsey KM, Courtney SC, Guo Z, Kapczynski DR: Effects of 

Newcastle disease virus vaccine antibodies on the shedding and transmission of challenge 

viruses. Developmental & Comparative Immunology 2013, 41(4):505-513

Spackman E, Pedersen JC, McKinley ET, Gelb J: Optimal specimen collection and transport 

methods for the detection of avian influenza virus and Newcastle disease virus. BMC 

veterinary research 2013, 9(1):35

Stevenson, M.A., Sanson, R.L., Stern, M.W., O’Leary, B.D., Sujau, M., Moles-Benfell, N., Morris, R.S. 

(2013). Interspread plus: A spatial and stochastic simulation model of disease in animal 

populations. Preventive Veterinary Medicine, 109(1-2), 10-24. doi: 

10.1016/j.prevetmed.2012.08.015

Ward, M. (2007). Spatiotemporal analysis of infectious disease outbreaks in veterinary 

medicine: clusters, hotspots, and foci. Vet Ital. 43(3), 559-570.

Wise MG, Suarez DL, Seal BS, Pedersen JC, Senne DA, King DJ, Kapczynski DR, Spackman E: 

Development of a real-time reverse-transcription PCR for detection of Newcastle disease 

virus RNA in clinical samples. Journal of Clinical Microbiology 2004, 42(1):329-338
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VI. ACKNOWLEDGEMENTS

We greatly appreciate the cooperation and support of the poultry owners for allowing us access 

to their properties, providing information on their biosecurity and production practices, and for 

their cooperation with this report. 

We also appreciate the Federal and State animal health experts, as well as the many partners 

who assisted with this study while serving in the incident command for this outbreak. 

This report would not have been possible without a large group of researchers, epidemiologists, 

laboratory staff, and data scientists who participated in the various analyses, and the drafting 

and review of this report. We appreciate their dedication and professionalism in providing the 

best information possible to help bird owners reduce the risk of Newcastle disease spread.
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APPENDIX A: QUESTIONNAIRE

California Department of Food and Agriculture

CA VND 2018

Non-Commercial Premises 

Virulent Newcastle Disease Epidemiology Questionnaire

Investigator name: ____________________Date of Investigation: _______/_____/_____

Investigator name: ____________________

Quarantine # ______________________ Date Quarantine Issued: _____/_____/_____

1. Name of Premises Owner:

___________________________________________

(First) (MI) (Last)

2. Premises Address (location of birds):

________________________________________________

________________________________________________

Latitude: ___________________ Longitude: ___________________

3. Premises Owner Telephone #: 

a. Mobile: ___________________

b. Home: ___________________

c. Other: ___________________

If Premises Owner is the Bird Owner skip to Question 7

4. Name of Bird Owner:

___________________________________________

(First) (MI) (Last)

5. Bird Owner Address: ________________________________________________

_______________________________________________

6. Bird Owner Telephone #: ______________________

7. Other than the interviewee, how many other owners with birds 

are on this premises: #_______
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8. How many birds do you have on the premises today? # ______________

9. What percent of the adult chickens are: a) Roosters %____________________

b) Hens %_______________________

10. Which of the following birds are on the premises? Complete table below.

Type of Bird # Adults # Young birds Total

Backyard Poultry a b c

Exhibition Birds/gamefowl d e f

Ducks/Geese g h i

Other 

Specify j k l m

Other 

Specify n o p q

11. Which of the following animals are on the premises (potential fomites)? 

a) Livestock (Horses, Cattle, Swine, Sheep, Goats) 1 Yes 3 No

b) Dogs/Cats 1 Yes 3 No 

c) Other (specify _____________________) 1 Yes 3 No

12. Which of the following housing types are used to house birds? 

a) Inside the home 1 Yes 3 No

b) Outdoor open top poultry pen or enclosure 1 Yes 3 No

c) Outdoor cages or coops - fully enclosed 1 Yes 3 No

d) Individually tethered 1 Yes 3 No

e) Free range 1 Yes 3 No

f) Other (Specify__________________________) 1 Yes 3 No

13. Has there been an increase in illness in your birds 

on your premises? 1 Yes 3 No

a) If yes, how many days ago did the birds first show

signs of illness: ________days
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Which of the following clinical signs of illness have you observed? 

Check all that apply.

b) Not eating 1 Yes 3 No

c) Coughing/gasping 1 Yes 3 No

d) Depressed 1 Yes 3 No

e) Twisting of the neck 1 Yes 3 No 

f) Paralysis 1 Yes 3 No

g) Diarrhea 1 Yes 3 No

h) Swellings around the eyes and neck 1 Yes 3 No

i) Sudden death 1 Yes 3 No

j) Other (specify___________________) 1 Yes 3 No

14. Have there been any deaths in your birds on this premises 

during the past 30 days? 1 Yes 3 No

a) If yes, when did the first bird die? _____/___/______

b) If yes, how many birds died in the first 7 days? # ______________

c) If yes, how many birds have died in the past 7 days? # ______________ 

15. Do you keep any birds at another premises? 1 Yes 3 No

a) If yes, where are the birds housed?

________________________________________________

________________________________________________

16. Have you brought new birds onto this premises 

during the past 30 days? 1 Yes 3 No 

If Yes, list date and name the source and location of the new birds: 

 Date Source/Location

 

___/___/___a _________________________________b

___/___/___c _________________________________d

___/___/___e _________________________________f
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17. Have any of the following had contact with your birds, feed or water sources on your property in the 

last 30 days? 

a) Wild birds (e.g., pigeons, doves, sparrows) 1 Yes 3 No

b) Neighborhood/community chickens 1 Yes 3 No

c) Wild animals 1 Yes 3 No

18. Have any of your birds left these premises

during the last 30 days? 1 Yes 3 No 

If Yes, for what purposes listed below were the birds moved?

Purpose Date Destination (City/State) # of birds

Sale a b c

Show d e f

Competition g h i

Veterinary care j k l

Gift/trade m n o p

Other 

Specify q r s t

If Yes, did any birds leave and then return to these premises? 1 Yes 3 No

19. Do you give away or sell eggs from this premises? 1 Yes 3 No 

20. Do your neighbors have birds? 1 Yes 3 No

If No, skip to Question 23. 

If Yes, please note location(s) on the map at the end of the questionnaire.

21. When not cooped, do your birds ever visit the neighbor’s property? 1 Yes 3 No

22. Do your neighbor’s birds ever come onto your property? 1 Yes 3 No

a) If Yes, do the neighbors birds have contact with your birds? 1 Yes 3 No

23. Do you have family members or close friends 

who own/keep birds? 1 Yes 3 No

If Yes, do any of the following situations occur (evaluating direction of exposure): 

a) Your family or friends handle birds 

when they visit. 1 Yes 3 No

b) When visiting family/friends do 

you handle their birds. 1 Yes 3 No
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24. What is the name and location of the store(s) where you get 

feed and supplies for your birds?

Name Location (City)

__________________________a _____________________________b

__________________________c _____________________________d

__________________________e _____________________________f

25. Have the birds on your premises today been vaccinated 

with Newcastle vaccine? 1 Yes 2 Unsure 3 No

Vaccine does not protect against disease!

a) If Yes, at what age(s) were your birds vaccinated with Newcastle 

 vaccine? 

26. Have you seen any dead wild birds on your premises 1 Yes 3 No

in the last 30 days?

If Yes, what type of wild bird(s)?

____________________________a _______________________________b

____________________________c_ _______________________________d

Additional comments, observations and leads:

Insert Google Maps Image of the premises or draw a map and specify bird 

locations. Please indicate which neighbors, if any, have birds.

I __________________________________certify that I have ________birds on / / @ __________

 (owner signature) (number) (date and time)
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APPENDIX B: MODELING SCENARIO DESIGN

Control Activities Associated with Respective Response Options

Scenario Design Overview

1

Day 1 through Day 161 

Identical response 

activities for all 4 

scenarios

Day 162 

75 days 

post-first 

Scenario 1: Response 

Option 1

1, 2, 4, 6, 9, 11, 12, 14, 

16, 17, 19, 22, 24, 25, 26

Scenario 2: Response 

Option 2 

Scenario 3: Response 

Option 3 

Scenario 4: Response 

Option 4 

1, 2, 4, 7, 9, 11, 13, 14, 

16, 17, 20, 22, 24, 25, 26

1, 2, 3, 4, 7, 9, 11, 13, 14, 

16, 17, 18, 20, 23, 24, 

25, 26

1, 2, 3, 4, 5, 8, 10, 11, 13, 

15, 16, 17, 18, 21, 23, 

24, 25, 26

Specific Activities From 

Table B.1

Alternative Response 

Options Initial Response 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

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Table B.1 Individual Control Activities Included In Response Options Response 

Options

Control Activity 1 2 3 4

1 Depopulation: detected backyard premises x x x x

2 Depopulation: backyard premises in proximity to detected backyard premises in Muscoy x x x x

3 Depopulation: backyard premises in proximity to detected backyard premises in other 

high-risk zones (e.g.,Bloomington, Fontana, Riverside)

x x

4 Depopulation: detected commercial premises x x x x

5 Depopulation: backyard premises in proximity to all detected backyard premises x

6 Depopulation: capacity (low – maximum of 6 backyard premises per day) x

7 Depopulation: capacity (medium – maximum of 10 backyard premises per day) x x

8 Depopulation: capacity (high – maximum of 30 backyard premises per day) x

9 Movement restrictions for live animal movements originating from zoned backyard 

premises (low capacity – 30% of high capacity)

x x x

10 Movement restrictions enhanced for live animal movements originating from zoned 

backyard premises (high capacity)

x

11 Movement restrictions for live animal movements originating from zoned commercial x x x x

12 Movement restrictions for live animal movements originating from traced premises (low 

capacity – 50% of high capacity)

x

13 Movement restrictions for live animal movements originating from traced premises (high x x x

14 Surveillance – Passive: Sick calls – initiates active surveillance visit (low public disease 

awareness)

x x x

15 Surveillance – Passive: Sick calls – initiates active surveillance visit (high public disease 

awareness – results in greater number of calls and surveillance visits) 

x

16 Surveillance – Active: 1-km radial zone around detected backyard premises x x x x

17 Surveillance – Active: Irregular zone surge (Muscoy) [enhanced surveillance for backyard 

premises]

x x x x

18 Surveillance – Active: Irregular zone surge (e.g., Bloomington, Fontana, Riverside) 

[enhanced surveillance for backyard premises]

x x

19 Surveillance – Active (post-irregular zone surge): Low capacity – fewest number of 

backyard premises eligible for surveillance (approx. 30% of high capacity)

x

20 Surveillance – Active (post-irregular zone surge): Medium capacity (approx. 67% of high 

capacity)

x x

21 Surveillance – Active (post-irregular zone surge): High capacity – greatest number of 

backyard premises eligible for surveillance

x

22 Surveillance – Active: baseline response time after zone formation (approx. 2X longer 

response time tnan for enhanced response)

x x

23 Surveillance – Active: enhanced response time after zone formation x x

24 Surveillance – Active: weekly to bi-weekly sampling of commercial premises x x x x

25 Tracing live animal movements originating from detected farms (movements occurred 

prior to detection)

x x x x

26 Tracing indirect contacts originating from detected commercial farms (movements 

occurred prior to detection)

x x x x
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Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties December 2018

USDA APHIS VS 41

Epidemiologic Analyses of 

Virulent Newcastle Disease in 

Backyard Birds in California

December 2018 Report

USDA  APHIS  VS

Science, Technology, and Analysis Services

Center for Epidemiology and Animal Health

2150 Centre Avenue, Bldg. B, Fort Collins, CO, 80526-8117

Web: http://www.aphis.usda.gov/animal-health/ceah